The present invention relates to implantable defibrillation leads and electrodes, and more particularly to methods for the transvenous placement of defibrillation electrodes.
A defibrillation device provides an electrical stimulus to a patient in an area near, on or in the heart for the purpose of reviving a heart that is not beating in a manner sufficient to support life. While there are numerous medical terms that can be used to describe such a heart, such as cardiac arrest, ventricular fibrillation, and asystole, and while each term has a somewhat different technical meaning, all are serious conditions that must be corrected immediately to prevent death of the patient. Hence, a defibrillation device is used in an attempt to get the heart beating again. To this end, a high energy stimulation pulse is delivered to or near the heart through one or more defibrillation leads, each lead having one or more electrodes at the distal end thereof. The present invention is concerned primarily with defibrillation leads, and with a manner of positioning the electrodes of such leads on or near the heart so that they can provide the greatest benefit.
Early defibrillation devices were large and cumbersome units that included a set of paddles, connected to a source of stored electrical energy through large wires. The paddles were positioned on the chest of the patient, typically by a doctor or paramedic, and the stored electrical energy was discharged one or more times through the paddles into the patient's body tissue. While such large defibrillation devices provided, and continue to provide, a measure of life support in emergency situations, such support can only be provided if a physician or paramedic having access to such a device is present.
It was recognized early that a defibrillation device could be carried by the patient at all times, i.e., the defibrillation device could be made portable and adapted to respond automatically to a stopped heart. With such a portable device, the needed life-sustaining defibrillation pulses could be automatically provided to the patient even in the absence of a physician or paramedic. One such early portable defibrillation device is disclosed in U.S. Pat. No. 3,866,615. The '615 patent teaches a light weight, portable cardiac emergency stimulator that includes separate defibrillation and pacemaker electronic circuits. The leads and electrodes used with the portable device are introduced into the patient's heart by a needle through the chest wall.
Implantable defibrillation devices have also been developed, as shown in U.S. Pat. No. 3,942,536. Such devices offer the advantages of the portable device without the need for introducing leads through the chest wall. In the '536 patent, defibrillation leads having endocardial electrodes are introduced transvenously into the inside of the heart, similar to the leads used by implantable pacemakers. Other attempts at using transvenous defibrillation leads having endocardial electrodes have also been made, as shown for example in U.S. Pat. Nos. 4,161,952 and 4,355,646.
The advantages of providing an implantable automatic defibrillator in certain patients at high risk of experiencing ventricular fibrillation or other heart disorders are thus readily apparent. When fibrillation or related heart malfunctions are sensed by such devices, a large defibrillation shock is automatically delivered to the heart in an attempt to stimulate the heart back to a normal or near normal beating pattern. The life-saving defibrillation shocks are delivered without any undue delay, as would otherwise exist if external defibrillation pulses had to be delivered by paramedics (or other medical personnel) who were summoned to the aid of a heart-failing patient.
One of the main problems associated with defibrillating a heart (replacing a dangerous rhythm with a more normal one) with an electrical stimulus, however, is that a relatively large surface area of the myocardial tissue, typically ventricular myocardial tissue, must be stimulated in order to overcome fibrillation. This problem is compounded by the fact that the myocardium, which comprises mostly cardiac muscle, is the middle of three layers of tissue that comprise the heart wall, the inner layer being termed the endocardium, and the outer layer being termed the epicardium. Hence, the myocardium is generally not directly accessible with a defibrillation electrode. Rather, the defibrillation pulse (electric field potential) must pass through one or more layers of other tissue before reaching the myocardial muscle tissue that needs to be depolarized (excited). Some of the energy is naturally expended on body fluids and tissues other than the myocardium. Hence, more energy must usually be delivered over a larger tissue area than would otherwise be required if the myocardial tissue were more directly accessible.
Prior art defibrillation leads and electrodes have generally been concerned with the size and shape of the surface area of the electrodes and correctly positioning the electrodes relative to the heart. Typically, at least a pair of such electrodes are positioned on or in the patient so that the defibrillating electrical energy passes through the appropriate myocardial tissue and the amount of energy delivered to other tissues is minimized. U.S. Pat. Nos. 4,030,509; 4,291,707; and 4,548,203 are representative of such efforts. Unfortunately, placement of relatively large electrodes on the exterior of the heart, i.e., epicardial electrodes, has usually required open chest surgery--a difficult and somewhat risky procedure at best. Placement of large electrodes on the interior of the heart, i.e., endocardial electrodes, is not easy without open-heart surgery--an even more difficult and risky procedure. Furthermore, placement of large electrodes within the heart may impair cardiac function and/or contribute to thrombosis or emboli formation in the left heart.
One problem associated with placement of epicardial defibrillation electrodes is that the heart resides in the pericardium. The pericardium is a membranous sac that encloses the heart. It consists of an outer layer of dense fibrous tissue, with an inner serous layer (the epicardium) which directly surrounds the heart. While it is possible, and sometimes preferred, to place defibrillation electrodes external to the pericardium, such placement typically requires an increased defibrillation energy as the electrical stimulus must pass through the pericardium and epicardium (and any other tissue in the electrical path) before reaching the myocardium. Hence, the amount of defibrillation energy required can be reduced somewhat if direct contact is made between the defibrillation electrode and epicardial tissue. However, before such direct contact can be made, the pericardium must somehow be pierced. Again, this has usually required open-chest surgery, although other techniques for gaining access to the heart have been proposed. See, e.g., U.S. Pat. No. 4,270,549; and applicant's copending U.S. patent application, "Sub-Xiphoid Positioning of Epicardial Defibrillation Electrodes and Electrode Anchoring Means," filed Apr. 4, 1989, as Ser. No. 07/333,391, which application is incorporated herein by reference.
Because of the problems associated with placement of epicardial electrodes, the concept of a transvenously implanted defibrillation lead and endocardial electrode remains an attractive alternative to open-chest surgery. Unfortunately, to date transvenous placement of defibrillation leads and electrodes (acting alone or in concert with subcutaneous electrodes) has proven unsatisfactory because the electrode surface area can not be made large enough for energy efficient cardiac defibrillation. Most prior uses of transvenous defibrillation leads with their resulting endocardial electrodes have thus been limited to uses in combination with epicardial electrodes, as shown for example in U.S. Pat. No. 4,641,656. (In this regard, it should be noted that the amount of energy required to defibrillate a typical fibrillating heart is much larger than the energy required to stimulate a non-fibrillating heart, as is used for example, by a pacemaker.) What is needed, therefore, is a technique for transvenously placing defibrillation leads having epicardial or pericardial electrodes thereby avoiding the trauma and potential problems of open chest surgery.
As indicated above, epicardial electrodes ar generally preferred because their use generally minimizes the energy of a defibrillation pulse, and thereby improves the efficacy of the defibrillation system. Epicardial electrodes are in direct contact with the heart tissue. Further, epicardial electrodes cover large and strategic areas of the heart, thereby allowing the delivered electrical energy to be efficiently distributed throughout the fibrillating region. Such epicardial electrodes are typically placed around the exterior of the heart within the pericardial space. Although there are some shortcomings associated with placement of defibrillation electrodes directly on the epicardial surface, the advantages are overwhelming.
In some situations, it may be preferred to place the electrodes on the outer surface of the pericardium, thereby avoiding the necessity of piercing the pericardium. While the energy delivered by such pericardial electrodes must pass through one additional layer of tissue (the pericardium), the pericardial electrodes are, in most other respects, just as advantageous as the epicardial electrodes. Because of the large surface area covered by many of these electrodes, they are sometimes referred to as "patch electrodes", often resembling patches that are placed on the heart.
Unfortunately, however, as has been indicated, pericardial or epicardial placement of defibrillation leads is a dangerous and difficult procedure that has heretofore generally required traumatic and endangering surgery, usually open-chest surgery. Needless to say, not all patients are suitable candidates for open-chest surgery, and even for those that are, the risks, trauma, and danger associated with such surgery make this procedure of electrode placement less than ideal. Hence, there is a need, as indicated above, for placement of pericardial electrodes on the pericardium, or for the placement of epicardial electrodes in the propitious pericardial space, without having to resort to dangerous open-chest surgery.
In an attempt to minimize the problems associated with open-chest surgery for the placement of epicardial defibrillation leads, it has been suggested to implant epicardial defibrillation leads transvenously. Such an approach is described in patent application Ser. No. 07/128,326, filed 12/03/87, now U.S. Pat. No. 4,884,567, entitled "Method for Transvenous Implantation of Objects into the Pericardial Space of Patients," of which the applicant named herein is a co-inventor. This prior application, including the methods and leads described therein (hereafter referred to as the "transvenous implantation approach"), is incorporated by reference herein.
In accordance with the transvenous implantation approach described in the abovereferenced prior application, a guide wire and a catheter are inserted into the heart transvenously, with the aid of an introducer, as required. Once in the heart, the right atrial lateral wall is punctured, making a hole therein, through which a non-deployed defibrillation electrode is inserted, thereby entering the pericardial space. The nondeployed electrode is further positioned within the pericardial space to a desired position, and then the electrode is deployed so as to better contact a larger surface area of the outside of the heart.
The transvenous implantation approach described in referenced document offers a very viable alternative to open chest surgery for many patients. However, for other patients, the risks and trauma associated with puncturing through the atrial wall, even though less than the risks and trauma associated with open-chest surgery, may not be acceptable, either because of the actual risks for a particular patient or because of the perceived risks. Hence, what is needed is a technique for placing pericardial or epicardial electrodes that not only avoids the need for dangerous open-chest surgery, but that also eliminates the risk and trauma that may accompany atrial puncture. The present invention advantageously addresses these and other needs.